1. Field of the Invention
The present invention relates to a process for the thermomechanical degradation of polyolefins in a twin-screw extruder without a downstream degradation apparatus, at from 300.degree. to 550.degree. C. and from 1 to 100 bar and in average residence times of the reaction mixture of from 0.5 to 10 minutes.
The present invention also relates to low molecular weight polyolefin waxes obtainable by the novel process and to their use as materials in cosmetics, in the surface coatings sector, in the printing sector and for toners.
2. Description of the Related Art
The preparation of polyolefin waxes, in particular polypropylene waxes, in a broad molecular weight range in combination with a narrow molecular weight distribution by controlled polymerization by an economical method has not been possible to date. Such waxes are therefore predominantly obtained by thermal degradation of high molecular weight polyolefins ((U.S. Pat. No. 3,087,922, FR-A 1 289 767, FR-A 1 252 635, FR-A 1 425 695, U.S. Pat. No. 3,519,609, U.S. Pat. No. 3,562,788, CA-A 797 293, DD-A 50 113).
DE-C 3 003 768 discloses a process for the preparation of polyolefin wax, in which a mixture of polyethylene and polypropylene is thermally degraded inside a heated reactor, in particular a heated metal tube. The heated metal tube is divided into two different zones which differ in that the reaction media are present in them in one case in a homogeneous state and in the other case in a heterogeneous state. In this process, the heat energy required for the thermal degradation is applied to the reaction medium by heating the two zones to different extents.
Furthermore, DE-B 1 940 686 describes a process for the continuous preparation of waxy, low molecular weight polyethylene from solid high molecular weight polyethylene, the high molecular weight polyethylene melted in an extruder being transferred to a heated tube where it undergoes thermal degradation. In this patent, it is stated, inter alia, that an additional pressure can be established at the end of the heated tube by means of a valve or a nozzle, against which pressure the polyethylene is extruded.
EP-A 0 474 889 likewise discloses a process for the thermal decomposition of polymers, in particular polyolefins, in which the polymer is first melted in an extruder and the molten polymer thus obtainable is then subjected to thermal decomposition in a tube reactor. The tube reactor used may also be provided with a static mixer in order thus to achieve more effective decomposition of the polymer.
The literature furthermore describes a process for the thermomechanical degradation of polyisobutylene/polystyrene blends [F. P. La Mantia, M. A. Nocilla in Polymer Degradation and Stability 17 (1987), 279-286]. This publication reveals in particular that the degradation of plastics blends can be influenced both by the supply of heat energy and by the supply of mechanical energy.
However, the waxes obtained in this process frequently have a number of disadvantages, for example the yellow coloration of the end product or high contents of byproducts, for example soot particles. In addition, the formation of wall deposits and caking are often observed in this process, particularly during continuous operation. In the preparation of very low molecular weight polyolefin waxes, very long residence times also generally have to be accepted, giving rise to high costs. Moreover, an increasing residence time, ie. an increasing degree of degradation, leads to increasing discoloration. Furthermore, the processes known to date often cannot be exactly controlled and lead to high contents of soot particles, especially in the degradation of polypropylene. The low molecular weight polypropylenes thus obtained frequently contain many, generally aperiodically distributed soot particles, which are disadvantageous with regard to subsequent use, for example in masterbatches.